1. Field of the Invention
This invention relates to electromagnetic compatibility (EMC) testing and, more particularly, to electric field generating devices, or energy transducers used for exposing devices under test to high-intensity electromagnetic fields over a large range of frequencies.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Electromagnetic energy is considered electromagnetic interference (EMI) when it adversely affects the performance of an electronic system. All electronic devices create some form of electromagnetic energy that potentially interferes with the operation of other electrical devices outside the system (inter-system) or within the system (intra-system). As such, all electronic devices are capable of interfering with other devices (emission), or being affected by the emissions from other devices through the transfer of electromagnetic energy. The transfer of electromagnetic energy may occur as conducted energy, radiated energy, or electrostatic discharge (ESD). Conducted interference is the transfer of energy between two or more conductive paths, whereas radiated interference is the transfer of energy through space and occurs by means of near- and/or far-field coupling. Electrostatic discharge, on the other hand, is the rapid transfer of electrostatic charge between bodies of different electrostatic potential, either in proximity in air (air discharge) or through direct contact (contact discharge).
Electromagnetic energy may also produce varying levels of interference. On a low interference level, EMI may produce “cross-talk” between conductive paths, which tends to increase the background noise level within signals traversing the paths. On the other hand, however, EMI can cause significant problems and even system failure in devices that are highly sensitive to electromagnetic radiation, such as automotive electronic systems (e.g. anti-lock braking systems).
Due to the problems created by EMI, allowable limits of EMI have been set at national and international levels. For example, the Federal Communications Commission (FCC) has set limits on the amount of electromagnetic radiation that is allowably emitted from commercial electronic equipment. As such, all commercial electronic equipment must be tested for electromagnetic compatibility (EMC) and must comply with the standards set by the Commission.
Electromagnetic compatibility relates to the capability of an electronic system to operate within its intended environment at desired levels of efficiency without causing or receiving degradation due to electromagnetic interference. As such, EMC typically includes both emissions testing (i.e., how emissions originating from a system interfere with another system) and immunity testing (i.e., how a system is affected by the emissions originating from another system).
EMC testing typically involves the generation of high-intensity electromagnetic fields over a wide range of frequencies to test for the possibility of isolated, narrow band phenomena which can take place anywhere over the frequency range. Though the frequency spectrum of electromagnetic energy can span from DC (0 Hz) to gamma ray frequencies (1012 Hz) and beyond, the frequency spectrum for use in EMC testing typically ranges from a few hertz (i.e., extreme low frequency, ELF) to approximately 40 GHz (i.e., microwave bands). This broadband generation of high-intensity electromagnetic fields typically presents a formidable challenge to designers.
In some cases, a conventional antenna may be used for electric field generation in EMC testing. However, an antenna may be subject to severe physical limitations, such as limited bandwidth, field pattern frequency dependency, and wide spatial variations in field intensity for a given frequency. In addition, antennas may require high input power to produce radiation at a distance suitable for convenient testing of a test device. Other types of electric field generators may be used to generate intense electromagnetic fields over a comparatively wider range of frequencies with relatively lower input power.
Electric field (“E-field”) generators typically fall into one of two categories. The first category is the unterminated, or open-circuit E-field generator, which generates an electric field between two parallel open-ended conductors in a capacitor-like fashion. The open-circuit E-field generator normally includes two spaced, parallel elements having centers connected to opposite terminals of a signal source, which in turn is connected to a resistive load. In this manner, a device under test (DUT) may be placed between (or possibly near) the parallel conductors to measure the effect of the generated electric field on the DUT. Although the open-circuit E-field generator may produce intense electric fields in the vicinity of the parallel conductors, it may not be capable of producing sufficient field intensities over a test volume large enough to accommodate a variety of DUT sizes. For example, the open-circuit E-field generator may not produce sufficient field intensities at a distance spaced away from the generator to accommodate a large DUT without dramatically increasing the size of the generator or the input power supplied to the generator.
In addition, open-circuit E-field generators are not particularly useful in broadband applications. For example, open-circuit E-field generators are subject to resonance modes as the test frequency approaches the point in which the length (L) of the parallel elements is equal to one quarter of a wavelength (i.e., L=λ/4). In fact, due to center loading of the parallel elements, the open-circuit generator tends to resonate well before the frequency at which the length of the elements equals a quarter wavelength (e.g., 70% of λ/4). In this manner, test frequencies near resonant modes, or frequencies that are odd multiples of a quarter wavelength, may effectively short-circuit the source and disable the generator. As such, open-circuit generators are not frequency independent, and cannot produce uniform electric fields over a continuous and wide range of frequencies.
Another category of E-field generators is the transmission line generator, otherwise called an “E/H field generator” due to the fact that it generates both electric (E) and magnetic (H) fields. A transmission line generator typically includes a source at one end of a two-conductor transmission line with a terminating load arranged at an opposite end. In this manner, the terminated E-field generator is not subject to the frequency dependence or bandwidth limitations which commonly plague open-circuit generators. In addition, a terminated E-field generator may advantageously decrease the amount of power reflected within the conductors by matching the impedance of the load to the characteristic impedance of the transmission line structure. For example, if the load impedance is a matched resistive load (i.e., has a resistance substantially equal to the resistance of the conductors), the load resistor will absorb the incident wave, so that no reflected wave will be generated at the load. In this manner, a well-matched system may have a return loss (i.e., the ratio of the reflected power to the incident power) of 15 dB or more, which corresponds to a voltage standing wave ratio (VSWR) of 1.43:1 or less. Though designers strive for a relatively low VSWR value (e.g., a VSWR of 1:1 corresponds to a perfectly matched system), a device may still function adequately even when it exhibits a 3 dB return loss, or a VSWR of 5.8:1. For practical purposes, however, designers typically strive for an impedance match that provides no more than 2:1 VSwR. For critical applications, it may be desired to achieve an impedance match of less than 1.5:1.
As in the case of the open-circuit generator, a disadvantage of the terminated E-field generator is that the generated electric field cannot be increased without increasing the size of the generator and/or the input power to the generator. An inherent property of wave propagation states that the intensity of the electric field decreases as the distance from the conductive elements increases. It can be shown, however, that by using the largest possible conductive elements along with the largest possible spacing between conductive elements, the electric field can be maximized at a given distance spaced from the conductive elements. In other words, the overall dimensions of the transmission line generator must be increased to obtain greater field intensities at distances spaced from the generator. However, the required size of the generator may surpass practical limitations (such as the size of a chamber enclosing the measurement) in the pursuit of adequate field intensities for testing larger electronic devices.
Therefore, it may be desired to provide an E-field generator which is capable of producing an increased electric field at a distance spaced from the generator without increasing the dimensions of the generator or the input power supplied to the generator. In addition, the desired generator will generate an intense, localized electric field substantially independent of frequency, and thus, may operate over a continuous broadband frequency range. Thus, for a given input power and test volume, the desired E-field generator will be capable of producing a significantly greater electric field than conventional generators of comparable dimensions.